When three digits can go only in the same three cells of a house, those cells must hold them, and their other candidates can go.
On a Kropki board this works exactly as in Classic. The examples are from Kropki puzzles on this site. Hard Kropki puzzles start with almost no clues, so it comes up often in them.
Worked examples
A position from a real Kropki puzzle on this site. Step through it: the board lights what each step talks about, and the arrows show what rules what out.
Three digits, three cells, in a row
In row 5, look for three digits that fit in only three cells.
Where it fits
HardHidden Triple is a Hard technique: Hard Kropki puzzles are the first to need it.
The Classic Hidden Triple lesson teaches it on a plain 9×9 board. The Kropki overview explains the rule and the levels.
Try it on a Hard Kropki puzzle. Next Step points it out when it is the move.